Add support for multi-allelic variants

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.14286% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 95.63%. Comparing base (9a2d510) to head (0232972).

Files	Patch %	Lines
.../scala/com/fulcrumgenomics/vcf/DownsampleVcf.scala	97.14%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## tb-add-downsample-vcf #976 +/- ## ========================================================= - Coverage 95.64% 95.63% -0.01% ========================================================= Files 127 127 Lines 7457 7473 +16 Branches 521 501 -20 ========================================================= + Hits 7132 7147 +15 - Misses 325 326 +1 ``` | [Flag](https://app.codecov.io/gh/fulcrumgenomics/fgbio/pull/976/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=fulcrumgenomics) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/fulcrumgenomics/fgbio/pull/976/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=fulcrumgenomics) | `95.63% <97.14%> (-0.01%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=fulcrumgenomics#carryforward-flags-in-the-pull-request-comment) to find out more.

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